Radiative Properties of Boundary Layer Clouds: Droplet Effective Radius versus Number Concentration

Abstract

The plane-parallel model for the parameterization of clouds in global climate models is examined in order to estimate the effects of the vertical profile of the microphysical parameters on radiative transfer calculations for extended boundary layer clouds. The vertically uniform model is thus compared to the adiabatic stratified one. The validation of the adiabatic model is based on simultaneous measurements of cloud microphysical parameters in situ and cloud radiative properties from above the cloud layer with a multispectral radiometer. In particular, the observations demonstrate that the dependency of cloud optical thickness on cloud geometrical thickness is larger than predicted with the vertically uniform model and that it is in agreement with the prediction of the adiabatic one. Numerical simulations of the radiative transfer have been performed to establish the equivalence between the two models in terms of the effective radius. They show that the equivalent effective radius of a vertically uniform model is between 80% and 100% of the effective radius at the top of an adiabatic stratified model. The relationship depends, in fact, upon the cloud geometrical thickness and droplet concentration. Remote sensing measurements of cloud radiances in the visible and near infrared are then examined at the scale of a cloud system for a marine case and the most polluted case sampled during the second Aerosol Characterization Experiment. The distributions of the measured values are significantly different between the two cases. This constitutes observational evidence of the aerosol indirect effect at the scale of a cloud system. Finally, the adiabatic stratified model is used to develop a procedure for the retrieval of cloud geometrical thickness and cloud droplet number concentration from the measurements of cloud radiances. It is applied to the marine and to the polluted cases. The retrieved values of droplet concentration are significantly underestimated with respect to the values measured in situ. Despite this discrepancy the procedure is efficient at distinguishing the difference between the two cases.

Examination of the aerosol indirect effect under contrasting environments during the ACE-2 experiment

“…The most extensive observations were conducted close to local noon (Brenguier et al, 2000a(Brenguier et al, , 2000bSnider et al, 2000;. The flight time, on 26 June was from 11:50 LST to 14:50 LST, while on 9 July, it was from 12:40 LST to 15:50 LST, and, our comparisons were conducted at the corresponding flight time.…”

Influence of aerosols on the shortwave cloud radiative forcing from North Pacific oceanic clouds: Results from the Cloud Indirect Forcing Experiment (CIFEX)

“…Figure 2 shows the probability densities of the ratio of observed LWC to LWC adiab , the expected adiabatic LWC computed according to Brenguier [1991], for four clean and three polluted clouds profiled during CIFEX on three different days (polluted is N a > 50 cm À3 below cloud base). Similar to the results from ACE-2 clouds [Brenguier et al, 2000], the densities peak at sub-adiabatic values, mostly associated with entrainment close to cloud top, although considerable scatter in the estimated ratio may also result from drizzle in some of the clean clouds. Furthermore, the flight tracks were typically diagonal traverses through the clouds where the horizontal distance traveled from cloud base to cloud top may have been broader than the horizontal scale of individual clouds, which will introduce some error in the estimate of LWC/LWC adiab , and may be the cause of the apparent super-adiabatic cases.…”

Determination of three‐dimensional cloud structures from high‐resolution radiance data

“…The variation of the cloud top height is characterized by standard deviations between 14 and 71 m depending on the cloud scene. Keeping in mind the limited number of cases the average numbers are in good agreement with the values measured in situ or via remote sensing techniques for the according dates of the ACE2 CLOUDYCOLUMN campaign [Brenguier et al, 2000a;Schüller et al, 2003]. The average value of LWP of 49 g/m 2 , including about 35% cloudless columns (at a resolution of 15 m  15 m), compares well to measurements of the LWP using microwave remote sensing during the Baltex Bridge Campaign (BBC) by Crewell et al [2004]: For nonprecipitating stratocumulus under marine influence an average LWP of 42 g/m 2 was measured.…”